1. Field of the Invention
The present invention relates to code division multiple access communication.
2. Description of the Prior Art
Code time division multiple access (CTDMA) has been proposed for cellular communications systems, for example mobile phone networks. In CTDMA systems, users in the same cell are allocated the same spreading code and respective time offsets. Although the users are using the same spreading code, orthogonality is presented because each user's use of the spreading sequence is time-shifted relative to the other users' use of it.
Under ideal conditions, many stations can use the same carrier frequency and spreading sequence because a one-chip offset between stations is sufficient to provide orthogonality. However, in real situations, signals often follow multiple paths of differing lengths between the transmitter and receiver. This causes interference when the path delay spread is greater than the spreading sequence offsets between users because the spreading sequence of the delayed signal from one station can become aligned with that of another at a receiver.